This invention relates to an optical pickup and hologram unit and, more particularly, to an optical pickup and hologram unit for diffracting reflection light from an optical disk (hereinafter, referred to merely as xe2x80x9cdiskxe2x80x9d), such as a CD or DVD, and converging it onto a light-receiving device.
The optical pickup for reading information out of a disk requires a function to reproduce recorded information as well as a function to detect focus error and tracking error. Conventionally, focus error has been detected by the well-known Foucault or astigmatism method while tracking error by the push-pull or three-beam method. Where using the Foucault and three-beam methods in combination, the light receiving elements 2a-2c for receiving a main beam and the light receiving elements 3a and 3b for receiving sub-beams have been employed as shown, for example, in FIG. 9. Focus error signals are determined from the difference of output signals between the light receiving elements 2a and 2b (S2axe2x88x92S2b). Tracking error signals are determined from the difference of output signals between the light receiving elements 3a and 3b (S3axe2x88x92S3b).
Because the sub-beam light reflected upon the disk passes obliquely through a lens, division is not equally two by the hologram unit 4 (FIG. 9) and hence the two sub-beams divided are not same in spot size. Consequently, the sub-beam diffracted as greater spot size (hereinafter, referred to as xe2x80x9caperture sidexe2x80x9d) after division and the sub-beam diffracted as smaller spot size (hereinafter, referred to as xe2x80x9cshade sidexe2x80x9d) are not in symmetry on a light receiving surface of the photodetector 1. On the light-receiving surface, the spot size of the shade-side sub-beam B is greater than the spot size of the aperture-side sub-beam A.
Meanwhile, in the conventional pattern design for a hologram unit 4, it has been emphasized that wavefront aberration be reduced for a main beam. However, large wavefront aberration remains left for sub-beams. That is, as shown in FIG. 10, conventionally a first pattern 6 and a second pattern 7 have been designed which are to be expressed as an even function Ax4+Bx2+C with respect to a distance x from a division line 5. These patterns have been divided into two and then joined together, thereby obtaining a whole pattern. In the pattern design, however, wavefront aberration has not been taken into consideration for sub-beams.
Due to this, there is a fear that the shade-side sub-beam B be possibly out of the light receiving elements 3a and 3b, as shown in FIG. 9. There has been a problem that the tracking error balance and jitter might vary significantly depending upon temperature change, resulting in unstable optical pickup characteristics.
Therefore, it is a primary object of this invention to provide an optical pickup capable of stabilizing the characteristics of the optical pickup.
A first invention is an optical pickup, comprising: a diffraction element for dividing a laser beam from a laser device into one main beam and two sub-beams; a lens for converging the main beam and the sub-beams onto a disk; a hologram unit divided into two of a first pattern and a second pattern to respectively diffract reflection light from the disk; and a photodetector including a first portion to receive the main beam and a second portion to receive the sub-beams respectively diffracted by the first pattern and the second pattern; wherein the first pattern and the second pattern in the hologram unit is made to minimize the spot of the sub-beam converging onto the second portion based on an optical length and a wavelength when the sub-beams are taken as light sources.
A second invention is a hologram unit, comprising: a hologram pattern for diffracting and converging onto a light receiving element two sub-beams reflected by a disk, the hologram pattern being made to minimize a spot of the sub-beams converging onto the second portion based on an optical path length and a wavelength when the sub-beams are taken as light sources.
Because the pattern on the hologram unit is designed to minimize a spot size of a sub-beam converging onto the light receiving device based on an optical path length and a wavelength when the sub-beam is taken as a light source, there is no fear that the sub-beam spot goes out of a light receiving surface of the light receiving device.
According to the invention, the optical pickup characteristics can be stabilized because the sub-beam spot can be prevented from going out of the light receiving surface.
The above described objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.